1. Field of the Invention
The present invention relates to an image forming apparatus and a sheet feeding method.
2. Description of the Related Art
Conventionally, in an image forming apparatus, correcting oblique-passing and adjusting timing (or so-called aligning) for sheets being fed are carried out by a regist roller or the like (see, for example, Jpn. Pat. Appln. Laid-Open Publication No. 10-167511). FIG. 7 shows a cross-sectional view indicative of the entire configuration of a conventional image forming apparatus.
In the image forming apparatus shown in FIG. 7, the following processing is performed. Firstly, sheets stocked in cassettes 31, 32, 33, and 34 are picked up by pickup rollers 31b, 32b, 33b, and 34b. Then, thus picked up sheets are separated to respective papers one by one by paper feeding rollers 31a, 32a, 33a, and 34a and separation rollers 31c, 32c, 33c, and 34c to be supplied to a sheet feeding path 35. Sheets fed to the sheet feeding path 35 are carried forward right before an intermediate feeding roller 38 by feeding rollers 36a to 36d. The intermediate feeding roller 38 is provided with a function of carrying forward sheets fed by an ADU 5, to be described later, or sheets fed on the sheet feeding path 35 to a regist roller 37 for correcting oblique-passing and adjusting timing for sheets.
After undergoing oblique-passing correction and timing adjustment, toner images formed on the photosensitive surface of a photosensitive drum 20 are transcribed to sheets carried forward to the regist roller 37 through the intermediate feeding roller 38. Toner images transcribed to sheets are fixed on the sheets by a fixing unit 24. Sheets having images fixed thereon are directly discharged to the outside of the image forming apparatus in case of one-side printing, and are carried forward to a reverse feeding path 28 in case of duplex printing or in case sheets that have undergone one-side printing are discharged after being reversed.
In case of duplex printing, sheets directed to the reverse feeding path 28 are made to switchback to be fed by a reverse rollers 29 and 30 to be sent to an ADU 5. Sheets sent to the ADU 5 are fed by feeding rollers 5a to 5d, and are directed to the intermediate feeding roller 38 again. Then, image forming processing by the photosensitive drum 20 and fixing unit 24 are carried out for the other sides of sheets fed through the ADU 5, realizing duplex printing for sheets.
FIG. 8 shows a view for explaining the sheet feeding right before the regist roller, which is an enlarged view of part enclosed by a dotted line in FIG. 7.
As shown in FIG. 8, in the image forming apparatus of above-described configuration, so as to correct oblique-passing and adjusting timing for sheets carried forward to the regist roller 37, the rotary drive of the regist roller 37 is stopped. Then, a sheet that is applied to a nip portion of the regist roller 37 is further pushed to bring about a deflection A by the intermediate feeding roller 38. After the deflection A is sufficiently raised, sheet feeding operation by the intermediate feeding roller 38 and the feeding roller 36a etc., located at the upstream of the intermediate feeding roller 38 is stopped to carry out aligning.
In case the feeding roller 36a etc., are of configuration in which motive power is transmitted through a clutch, there may be raised a delay in stopping feeding rollers due to a slip of the clutch and so on. This delay in the stop operation becomes the cause of occurrence of a deflection B right before the intermediate feeding roller 38. In general, the deflection A is larger than the deflection B.
In the conventional image forming apparatus of above-described configuration, when resuming the operation of feeding a sheet by the regist roller 37 after aligning the sheet, since the intermediate feeding roller 38 and the regist roller 37 are operated in synchronization with each other, sheet feeding speed of the intermediate feeding roller 38 and that of the regist roller 37 are equal to each other. So, the deflection B which is smaller than the deflection A raised at the time of aligning is necessarily eliminated faster than the deflection A. That is, the intermediate feeding roller 38 has to feed a sheet independently until the deflection A between the regist roller 37 and the intermediate feeding roller 38 is eliminated.
However, in case of aligning a sheet whose size is large in the sheet feeding direction (A3 size etc.), or in case a sheet is fed to the sheet feeding path 35 from a position near the intermediate feeding roller 38 (for example, in case a sheet is supplied from the cassettes 31, 32 of the upper stages), when the intermediate feeding roller 38 resumes the sheet feeding, the posterior edge of the sheet is pinched by the paper feeding rollers and the separation rollers whose retention torque is large, and there may be raised a case in which the intermediate feeding roller 38 (driven by a pulse motor and so on) whose torque is not large cannot feeding the sheet independently since the feeding load is too large (refer to a sheet P1 shown in FIG. 8). Such a problem is also raised in case a sheet is pinched by the feeding rollers 5c or 5d when the sheet is supplied to the sheet feeding path 35 from the ADU 5, or in case a sheet is pinched by many feeding rollers on the sheet feeding path 35 (refer to a sheet P2 indicated by a dotted line shown in FIG. 8). The above-described overload to the intermediate feeding roller 38 becomes the cause of failure such as step-out of an intermediate feeding motor for driving the intermediate feeding roller 38 and sheet jam.
The problem of lack of torque can be solved by driving the intermediate feeding roller 38 using a clutch to increase the torque, or by using a pulse motor of large torque, which undesirably prevents a reduction of cost and miniaturization of the device.